Thin film transistor and method for making the same

ABSTRACT

A thin film transistor is provided. The thin film transistor includes a source electrode, a drain electrode, a semiconducting layer, an insulating layer and a gate electrode. The insulating layer has a first surface and a second surface opposite to the first surface. The gate electrode is located on the first surface of the insulating layer. The source electrode, the drain electrode, and the semiconductor layer are located on the second surface of the insulating layer. The gate electrode, the source electrode, and the drain electrode include a first carbon nanotube layer. The semiconductor layer includes a second carbon nanotube layer. A first film resistor of the first carbon nanotube layer is smaller than or equal to 10 kΩ per square. A second film resistor of the second carbon nanotube layer is greater than or equal to 100 kΩ per square.

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201310130609.6, filed on Apr. 16, 2013 inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates to thin film transistors and,particularly, to a carbon nanotube based thin film transistor.

2. Description of Related Art

A typical thin film transistor (TFT) is made of a substrate, a gateelectrode, an insulation layer, a drain electrode, a source electrode,and a semiconductor layer. The thin film transistor performs a switchingoperation by modulating an amount of carriers accumulated in aninterface between the insulation layer and the semiconductor layer froman accumulation state to a depletion state, with applied voltage to thegate electrode, to change an amount of the current passing between thedrain electrode and the source electrode.

Material of semiconductor layer is semiconductive material and materialsof source electrode and drain electrode are metal materials. Becausematerials of source electrode and drain electrode are different frommaterial of semiconductor layer, interface barrier existed between thesemiconductor layer and the source electrode or the drain electrode hasan adversely effect to the property of thin film transistor.Furthermore, the thickness of the semiconductor layer, the sourceelectrode and the drain electrode is relatively large which affects thelight transmittance of the thin film transistor.

What is needed, therefore, is a thin film transistor that can overcomethe above-described shortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referencesto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a cross sectional view of one embodiment of a thin filmtransistor.

FIG. 2 is a schematic structural view of the thin film transistor ofFIG. 1.

FIG. 3 is a Scanning Electron Microscope (SEM) image of a first carbonnanotube layer.

FIG. 4 is an SEM image of a second carbon nanotube layer.

FIG. 5 is a test paragraph of on/off ratio of current of the thin filmtransistor of FIG. 1.

FIG. 6 is an Atomic Force Microscope (AFM) image of a first substratedeposited catalyst for growing the first carbon nanotube layer.

FIG. 7 is an AFM image of a second substrate deposited catalyst forgrowing the second carbon nanotube layer.

FIG. 8 is a cross sectional view of another embodiment of a thin filmtransistor.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

Referring to FIG. 1 and FIG. 2, a thin film transistor 10 of oneembodiment includes a gate electrode 120, an insulating layer 130, asemiconductor layer 140, a source electrode 150, and a drain electrode160. The insulating layer 130 has a first surface 132 and a secondsurface 134 opposite to the first surface 132. The gate electrode 120 islocated on the first surface 132. The semiconductor layer 140, thesource electrode 150, and the drain electrode 160 are located on thesecond surface 134. The thin film transistor 10 is positioned on asurface of an insulating substrate 110.

The source electrode 150 and the drain electrode 160 are spaced fromeach other. The semiconductor layer 140 is electrically connected to thesource electrode 150 and the drain electrode 160. The gate electrode 120is insulated from the semiconductor layer 140, the source electrode 150,and the drain electrode 160 through the insulating layer 130.

The thin film transistor 10 can be a bottom gate structure. In detail,the gate electrode 120 is located on the insulating substrate 110. Theinsulating layer 130 covers the gate electrode 120, and a part of theinsulating layer 130 is directly located on the insulating substrate110. The semiconductor layer 140 is located between and extends onto thesource electrode 150 and the drain electrode 160. A part of thesemiconductor layer 140 between the source electrode 150 and the drainelectrode 160 is defined as a middle part 142. The middle part 142 isdefined as a channel. A part of the semiconductor layer 140 overlappedwith the source electrode 150 is defined as a first connecting part 144.A part of the semiconductor layer 140 overlapped with the drainelectrode 160 is defined as a second connecting part 146. In oneembodiment, the first connecting part 144 is located on and in contactwith a surface of the source electrode 150 away from the insulatinglayer 130, and the second connecting part 146 is located on and incontact with a surface of the drain electrode 160 away from theinsulating layer 130.

The insulating substrate 110 is provided for supporting the thin filmtransistors 10. The material of the insulating substrate 110 can berigid materials, such as glass, crystal, ceramic, diamond, and silicon,or flexible materials such as plastic or resin. In detail, the flexiblematerial can be polycarbonate (PC), polymethyl methacrylate acrylic(PMMA), polyethylene terephthalate (PET), polyethersulfone (PES),cellulose ester, polyvinyl chloride (PVC), benzocyclobutenes (BCB),acrylic resins, acrylonitrile butadiene styrene (ABS), polyamide (PA),or combination thereof. In one embodiment, the material of theinsulating substrate is PET. The shape and size of the insulatingsubstrate 110 are arbitrary.

The gate electrode 120, the source electrode 150, and the drainelectrode 160 can include a first carbon nanotube layer. Referring toFIG. 3, the first carbon nanotube layer includes a plurality of firstcarbon nanotubes joined by van der Waals attractive force. The pluralityof first carbon nanotubes is single-walled carbon nanotubes and arrangeddisordered. The term ‘disordered’ is defined as the plurality of firstcarbon nanotubes is arranged along many different directions, and thealigning directions of the plurality of first carbon nanotubes arerandom. The plurality of first carbon nanotubes arranged along eachdifferent direction can be almost the same (e.g. uniformly disordered).The disordered first carbon nanotubes can be isotropic. The disorderedfirst carbon nanotubes entangle with each other to form the first carbonnanotube layer, and a plurality of apertures is defined by the pluralityof first carbon nanotubes. A diameter of the aperture can smaller than50 micrometers. The plurality of the apertures can enhance the lighttransparence of the first carbon nanotube layer.

The plurality of first carbon nanotubes is substantially parallel with asurface of the first carbon nanotube layer. The plurality of firstcarbon nanotubes has a large distribution density. The plurality offirst carbon nanotubes are connected with each other and form aconductive network. In one embodiment, number of the first carbonnanotubes in 1 square micrometers is equal to or greater than 20. Thenumber of carbon nanotubes in 1 square micrometers is defined asdistribution density. The first carbon nanotube layer has a small filmresistor and a great electric conductive property. In one embodiment,the first film resistor R_(s1) of the first carbon nanotube layer issmaller than or equal to 10 kΩ per square.

A diameter of first carbon nanotubes is smaller than about 10nanometers. A length of first carbon nanotubes ranges from about 1micrometer to about 2 millimeters. In one embodiment, the diameter offirst carbon nanotubes is about 6 nanometers, the length of first carbonnanotubes ranges from about 5 micrometers to about 100 micrometers.

In one embodiment, a thickness of the first carbon nanotube layer isabout 6 nanmometers, the first film resistor R_(s1) of the first carbonnanotube layer is about 5 kΩ per square, a length of the gate electrode120, the source electrode 150, and the drain electrode 160 is about 10micrometers, a width of the gate electrode 120, the source electrode150, and the drain electrode 160 is about 10 micrometers.

The semiconductor layer 140 can include a second carbon nanotube layer.Referring to FIG. 4, the second carbon nanotube layer includes aplurality of second carbon nanotubes joined by van der Waals attractiveforce. The plurality of second carbon nanotubes is single-walled carbonnanotubes and arranged disordered. The second carbon nanotube layer andthe first carbon nanotube layer have different distribution density ofcarbon nanotubes. Number of the plurality of second carbon nanotubes in1 per square micrometer is smaller than or equal to 1. The distributiondensity of the plurality of first carbon nanotubes is about 20 timesthat of the plurality of second carbon nanotubes. The second carbonnanotube layer has a larger film resistor than the first carbon nanotubelayer. In one embodiment, the second film resistor R_(s2) of the secondcarbon nanotube layer is greater than or equal to 100 kΩ per square.

A diameter of second carbon nanotubes is smaller than about 5nanometers. In one embodiment, the diameter of second carbon nanotubesis about 3 nanometers, a thickness of the second carbon nanotube layeris about 3 nanometers, and a length of the second carbon nanotubesranges from about 5 micrometers to about 100 micrometers.

A direction in a surface of the semiconductor layer 140 from the sourceelectrode 150 to the drain electrode 160 is defined as an X direction. Adirection in the surface of the semiconductor layer 140 thatsubstantially perpendicular to the X direction is defined as a Ydirection. A direction that substantially perpendicular to the surfaceof the semiconductor layer 140 is defined as a Z direction. A length ofthe middle part 142 of the semiconductor layer 140 along the X directionis defined as L. A width of the middle part 142 of the semiconductorlayer 140 along the Y direction is defined as W. A resistance R of themiddle part 142 of the semiconductor layer 140, the second film resistorR_(s2) of the second carbon nanotube layer, the length L of the middlepart 142, and the width W of the middle part 142 satisfy followingformula:

$R = {R_{s\; 2}\frac{L}{W}}$

A ratio of the length L and the width W of the middle part 142 isgreater than 1. The length L of the middle part 142 is greater than 5micrometers. The width W of the middle part 142 is equal to or greaterthan 5 micrometers. In one embodiment, the length L of the middle part142 is greater than or equal to 40 micrometers and smaller than or equalto 100 micrometers. The second film resistor R_(s2) of the second carbonnanotube layer is greater than or equal to 100 kΩ per square. Theresistance R of the middle part 142 is greater than or equal to 100 kΩ.The electric conductivity property of the middle part 142 is between aconductor and an insulator. The middle part 142 has a semiconductorproperty. In one embodiment, the length L of the middle part 142 isabout 40 micrometers, the width W of the middle part 142 is about 5micrometers, and the second film resistor R_(s2) of the second carbonnanotube layer is about 330 kΩ per square per square.

Length of the first connecting part 144 and the second connecting part146 along the X direction and the Y direction is arbitrary. In oneembodiment, length along the Y direction of the first connecting part144 and the second connecting part 146 is greater than the width alongthe Y direction of the middle part 142. In one embodiment, the lengthalong the Y direction of the first connecting part 144 and the secondconnecting part 146 is about 10 micrometers, and the width along the Xdirection of the first connecting part 144 and the second connectingpart 146 is about 5 micrometers.

The source electrode 150 and the drain electrode 160 are located on thetwo ends of the semiconductor layer 140 along the X direction. The firstconnecting part 144 is in contact with a part of the source electrode150. The second connecting part 146 is in contact with a part of thedrain electrode 160. Other part of the source electrode 150 and otherpart of the drain electrode 160 are exploded to connect with externalroute electrically (not shown). Because the thickness of thesemiconductor layer 140 is small, the semiconductor layer 140, thesource electrode 150 and the drain electrode 160 are located on a samesurface. The semiconductor layer 140 can only include the middle part142 with two ends in contact with and electrically connected to thesource electrode 150 and the drain electrode 160. Because each of thesemiconductor layer 140, the source electrode 150 and the drainelectrode 160 includes carbon nanotubes, a great interface between thesemiconductor layer 140 and the source electrode 150 or the drainelectrode 160 is formed, therefore enhancing the property of the thinfilm transistor 10.

The material of the insulating layer 130 can be a rigid material such asaluminum oxide (Al₂O₃), silicon nitride (Si₃N₄), silicon dioxide (SiO₂),or a flexible material such as polyethylene terephthalate (PET),benzocyclobutenes (BCB), polyester or acrylic resins. A thickness of theinsulating layer 130 can be in a range from about 10 nanometers to about100 micrometers. In one embodiment, the material of the insulating layer130 is Al₂O₃, and the thickness of the insulating layer 130 is about 40nanometers.

In use, the source electrode 150 is grounded. A voltage Vds is appliedto the drain electrode 160. Another voltage Vg is applied on the gateelectrode 120. The voltage Vg forming an electric field in the channel.Accordingly, carriers exist in the channel near the gate electrode 120.As the Vg increasing, a current is generated and flows through thechannel. Thus, the source electrode 150 and the drain electrode 160 areelectrically connected. Referring to FIG. 5, the thin film transistor 10has high on/off ratio of current (>10⁴) and electron mobility.

A method of making the thin film transistor is further provided. Themethod includes following steps:

-   -   Step (S1), providing an insulating substrate 110;    -   Step (S2), forming a gate electrode 120 on the insulating        substrate 110, wherein the gate electrode 120 includes a first        carbon nanotube layer with a first film resistor smaller than or        equal to 10 kΩ per square;    -   Step (S3), forming an insulating layer 130 on the gate electrode        120;    -   Step (S4), forming a source electrode 150 and a drain electrode        160 on the insulating layer 130, wherein the source electrode        150 and the drain electrode 160 are spaced from each other, the        source electrode 150 and the drain electrode 160 include a first        carbon nanotube layer with a first film resistor smaller than or        equal to 10 kΩ per square per square; and    -   Step (S5), forming a semiconductor layer 140 on the insulating        layer 130, wherein the semiconductor layer 140 is in contact        with the source electrode 150 and the drain electrode 160, and        the semiconductor layer 140 includes a second carbon nanotube        layer with a second film resistor greater than or equal to 100        kΩ per square per square.

In step (S1), the insulating substrate 110 can be rigid materials, suchas glass, crystal, ceramic, diamond, and silicon, or flexible materialssuch as plastic or resin. In one embodiment, the material of theinsulating substrate is polyethylene terephthalate. The insulatingsubstrate 110 can be further hydrophilic treated.

In step (S2), the first carbon nanotube layer includes a plurality offirst carbon nanotubes. The first carbon nanotube layer can be anintegrity of the plurality of first carbon nanotubes arrangeddisordered. The method of forming the gate electrode 120 is as follows:

-   -   Step (S21), providing a first substrate and pre-treating the        first substrate;    -   Step (S22), depositing a first catalyst layer on a surface of        the first substrate;    -   Step (S23), placing the first substrate with the first catalyst        layer in a reactor, and inputting an inert gas to the reactor;    -   Step (S24), inputting a carbon source gas, and growing the first        carbon nanotube layer in a stepped temperature manner via        chemical vapor deposition; and    -   Step (S25), transferring the first carbon nanotube layer from        the first substrate to the insulating substrate 110 and forming        a gate electrode 120.

In step (S21), the first substrate can be further hydrophilic treatedfor combining with the first catalyst layer better. In one embodiment,the first substrate is pre-treated by following steps: first,hydrophilic-treating the first substrate by hydrogen peroxide andammonia water; second, treating the first substrate by an organicsolvent. After treating by hydrogen peroxide and ammonia water, asurface of the first substrate has a plurality of hydroxyl groupscombining with the organic solvent. The organic solvent can include(3-aminopropyl)-triethoxysilane (APTES) with amino group to fix thefirst catalyst layer.

In step (S22), the first catalyst layer includes ferritin, metal, suchas iron, cobalt, nickel, or metal oxide. The first catalyst layer can beformed by electron beam evaporation or other electrochemical method. Inone embodiment, the first catalyst layer is ferritin. The first catalystlayer is deposited on the first subsrate by following steps: firstly, afirst ferritin solvent is obtained by mixing ferritin and water with afirst volume ratio; secondly, immersing the first substrate into thefirst ferritin solvent for a first time. The first volume ratio offerritin and water can be in a range from about 1:100 to about 1:20. Thefirst time of immersion can be in a range from 20 minutes to 2 hours. Inone embodiment, the first volume ratio of ferritin and water is about1:50, and the first time of immersion is about 1 hour. Referring to FIG.6, the image is taken from a region of a length of about 1 micrometerand a width of about 1 micrometer, the first catalyst layer includes aplurality of dense catalyst particles, and number of catalyst particlesof the first catalyst layer in 1 square micrometers is greater than 800.Because the distribution density of catalyst particles of the firstcatalyst layer is large, the plurality of first carbon nanotubes has alarge distribution density and forms an electric conductive network.

In step (S23), inputting the inert gas to the reactor is to make surethat environment of the reactor has no oxygen. The inert gas can benitrogen or argon. In one embodiment, the inert gas is argon.

In step (S24), the carbon source gas can be methane, methanol, ethanol,ethylene, or acetylene. The stepped temperature manner is as follows:step (a), oxidizing a plurality of catalyst particles of the firstcatalyst layer in air atmosphere under a first temperature for 5 minutesto 30 minutes; step (b), reducing the plurality of catalyst particles inhydrogen or ammonia atmosphere under a second temperature for 5 minutesto 30 minutes; step (c), inputting the carbon source gas under a thirdtemperature for a growing time and growing the plurality of first carbonnanotubes. The growing time of making the first carbon nanotube layer isin a range from about 20 minutes to about 2 hours. The first temperatureis in a range from about 650 degrees centigrade to about 750 degreescentigrade. The second temperature is in a range from about 750 degreescentigrade to about 850 degrees centigrade. The third temperature is ina range from about 850 degrees centigrade to about 950 degreescentigrade. Number of single-walled carbon nanotube of the first carbonnanotube layer in 1 square micrometers is greater than 20. In oneembodiment, the carbon source gas is a mixed gas of ethanol, methanoland methane with a carrier gas of hydrogen. A volume ratio of methanoland ethanol is in a range from about 1:1 to about 1:5. In oneembodiment, the volume ratio of methanol and ethanol is about 1:3. Aflowing rate of the carrier gas is in a range from about 50standard-state cubic centimeter per minute (sccm) to about 200 sccm. Aflowing rate of methane gas is in a range from about 50 sccm to about200 sccm. In one embodiment, the flowing rate of the carrier gas isabout 100 sccm, the flowing rate of methane gas is about 100 sccm, thestepped temperature manner includes: step (a) is under about 700 degreescentigrade for about 20 minutes, step (b) is under about 800 degreescentigrade for about 20 minutes, and step (c) is under the thirdtemperature is about 900 degrees centigrade for about 1 hour, the firstfilm resistor of the first carbon nanotube layer is about 5 kΩ persquare per square, and a thickness of the first carbon nanotube layer isabout 6 nanometers.

In step (S25), the first carbon nanotube layer is transferred as awhole. The first carbon nanotube layer can be transferred by a drytransferring method or a wet transferring method. The dry transferringmethod includes following steps: coating the first carbon nanotube layerby an adhesive belt; and detaching the adhesive belt via heating under atemperature from about 90 degrees centigrade to 150 degrees centigrade.In one embodiment, the adhesive belt is detached via heating under about120 degrees centigrade. The wet transferring method includes followingsteps:

-   -   (a1) coating the organic adhesive layer on the first carbon        nanotube layer and solidifying the organic adhesive layer;    -   (b1) detaching the first single-walled carbon nanotube layer        from the first substrate onto the organic adhesive layer via a        reactive reagent;    -   (c1) placing the first carbon nanotube layer and the organic        adhesive layer on the insulating substrate 110; and    -   (d1) removing the organic adhesive layer by an organic solvent.

The material of the organic adhesive layer can be positive photoresistZEP or polymethylmethacrylate (PMMA). The reactive reagent can reactwith the first substrate to separate the first carbon nanotube layerfrom the first substrate. The reactive reagent can be hydrogen fluoride,carbon fluoride, sodium hydroxide, or potassium hydroxide. In oneembodiment, the organic adhesive layer is PMMA. The first carbonnanotube layer can maintain the original structure by the organicadhesive layer.

After transferring, the first carbon nanotube layer can be etched toform the gate electrode 120. The method of etching the first carbonnanotube layer can be photolithography method, reactive ion etchingmethod (RIE), or oxidation method. In one embodiment, etching the firstcarbon nanotube layer includes following steps:

-   -   (a2) forming a mask on a surface of the first carbon nanotube        layer, wherein the mask is a hydrogen silsesquioxane layer;    -   (b2) etching part of the mask to obtain a mask with a pattern        and exposing part of the first carbon nanotube layer;    -   (c2) removing the exposed first carbon nanotube layer via RIE        method; and    -   (d2) removing the mask.

In step (S4), the insulating layer 130 can be formed via evaporating,sputtering, or atom layer depositing. In one embodiment, the insulatinglayer 130 is formed on a surface of the gate electrode 120 away from theinsulating substrate 110 via evaporating, the gate electrode 120 iscovered by the insulating layer 130 totally, material of the insulatinglayer 130 is aluminum oxide, and a thickness of the insulating layer 130is about 40 nanometers.

In step (S4), forming the source electrode 150 and the drain electrode160 includes following steps: forming the first carbon nanotube layer ona surface of the insulating layer 130, and etching the first carbonnanotube layer. Etching the first carbon nanotube layer is same as thatof step (S3). In one embodiment, the length along the Y direction of thesource electrode 150 and the drain electrode 160 is about 10micrometers, and the width along the X direction of the source electrode150 and the drain electrode 160 is about 5 micrometers.

In step (S5), making the second carbon nanotube layer is simple asmaking the first carbon nanotube layer of step (S2). The difference ofstep (S5) and step (S2) is that the volume ratio of ferritin and waterand the growing time. The volume ratio of ferritin and water of makingthe second carbon nanotube layer is in a range from about 1:8000 toabout 1:1000. The growing time of making the second carbon nanotubelayer is in a range from about 30 seconds to about 5 minutes. Number ofthe plurality of second carbon nanotubes in 1 square micrometers issmaller than 1. Therefore, the second carbon nanotube layer has asemi-conductive property. Referring to FIG. 7, the image is taken from aregion of a length of about 1 micrometer and a width of about 1micrometer, the second catalyst layer includes a plurality of secondcatalyst particles, and number of second catalyst particles in 1 squaremicrometers is about 60. In one embodiment, the volume ratio of ferritinand water of making the second carbon nanotube layer is about 1:2000,the growing time is about 1 minute, and second film resistor of thesecond carbon nanotube layer is about 330 kΩ per square per square, anda thickness of the second carbon nanotube layer is about 3 nanometers.

Part of the second carbon nanotube layer can further be etched to obtainthe semiconductor layer 140. The method of etching part of the secondcarbon nanotube layer is same as that of etching the first carbonnanotube layer of step (S4). The semiconductor layer 140 is formed byselectively etching part of the second carbon nanotube layer between thesource electrode 150 and the drain electrode 160. A ratio of the lengthand the width of the middle part 142 is greater than 1. The length ofthe channel is greater than 5 micrometers. The width of the middle part142 is equal to or greater than 5 micrometers. The middle part 142 hasgreat semiconductor property.

The second carbon nanotube layer cannot be etched so that a large firstconnecting part 144 and a large second connecting part 146 are formed.The large first connecting part 144 and large second connecting part 146can be in firmly contact with the source electrode 150 and the drainelectrode 160 respectively.

In one embodiment, the length L of the middle part 142 is about 40micrometers, the width W of the middle part 142 is about 5 micrometers,the length along the Y direction of the first connecting part 144 andthe second connecting part 146 is about 10 micrometers, and the widthalong the X direction of the first connecting part 144 and the secondconnecting part 146 is about 5 micrometers.

In the method for making the thin film transistor 10, because thedistribution density of the plurality of second carbon nanotubes issmaller than that of the plurality of first carbon nanotubes, the step(S4) of forming the source electrode 150 and the drain electrode 160 arebefore the step (S8) of forming the semiconductor layer 140, in order toavoid the second carbon nanotube layer being etched excessively. In oneembodiment, the source electrode 150 and the drain electrode 160 areformed at first, then the second carbon nanotube layer is placed on thesource electrode 150 and the drain electrode 160, part of the secondcarbon nanotube layer is etched on the insulating layer 13, and thechannel is obtained.

Referring to FIG. 8, a thin film transistor 10 of another embodimentincludes a gate electrode 120, an insulating layer 130, a semiconductorlayer 140, a source electrode 150, and a drain electrode 160. The sourceelectrode 150 and the drain electrode 160 are spaced from each other.The semiconductor layer 140 is electrically connected to the sourceelectrode 150 and the drain electrode 160. The gate electrode 120 isinsulated from the semiconductor layer 140, the source electrode 150,and the drain electrode 160 through the insulating layer 130. The thinfilm transistor 20 is positioned on a surface of an insulating substrate110.

The structure of the thin film transistor 20 is similar with that of thethin film transistor 10. The difference between the thin film transistor20 and the thin film transistor 10 is that the source electrode 150 andthe drain electrode 160 are located on the insulating substrate 110, thesemiconductor layer 140 is located on the insulating substrate 110 andextends onto the source electrode 150 and the drain electrode 160, andthe insulating layer 130 covers the semiconductor layer 140, the sourceelectrode 150, and the drain electrode 160. The gate electrode 120 islocated on a surface of the insulating layer 130 away from theinsulating substrate 110. The semiconductor layer includes a middle part142. The middle part 142 is between the source electrode 150 and thedrain electrode 160.

The thin film transistor has following advantages. Firstly, the materialof the semiconductor layer 140, the source electrode 150, and the drainelectrode 160 is carbon nanotubes, the interface resistance between thesemiconductor layer 140 and the source electrode 150 or the drainelectrode 160 is reduced, and on/off ratio of current and electronmobility of the thin film transistor is enhanced. Secondly, the diameterof the single-walled carbon nanotube is less than 10 nanometers, thethickness of the first carbon nanotube layer and the second carbonnanotube layer is small, thus the light transmittance of the thin filmtransistor is high.

Depending on the embodiments, certain of the steps described may beremoved, others may be added, and the sequence of steps may be altered.It is also to be understood that the description and the claims drawn toa method may include some indication in reference to certain steps.However, the indication used is only to be viewed for identificationpurposes and not as a suggestion as to an order for the steps.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present embodiments have been setforth in the foregoing description, together with details of thestructures and functions of the embodiments, the disclosure isillustrative only, and changes may be made in detail, especially inmatters of shape, size, and arrangement of parts within the principlesof the disclosure.

What is claimed is:
 1. A thin film transistor comprising: an insulatinglayer having a first surface and a second surface; a gate electrodelocated on the first surface of the insulating layer; a sourceelectrode; a drain electrode spaced from the source electrode; and asemiconductor layer electrically connected with the source electrode andthe drain electrode; wherein the source electrode, the drain electrode,and the semiconductor layer are located on the second surface of theinsulating layer; the gate electrode, the source electrode, and thedrain electrode comprise a first carbon nanotube layer; and thesemiconductor layer comprises a second carbon nanotube layer, a firstfilm resistor of the first carbon nanotube layer is smaller than orequal to 10 kΩ per square, and a second film resistor of the secondcarbon nanotube layer is greater than or equal to 100 kΩ per square. 2.The thin film transistor of claim 1, wherein the semiconductor layer islocated between the drain electrode and the source electrode.
 3. Thethin film transistor of claim 1, wherein the semiconductor layercomprises a middle part, a first connecting part, and a secondconnecting part; and the middle part is located between the firstconnecting part and the second connecting part and is defined as achannel.
 4. The thin film transistor of claim 3, wherein the firstconnecting part is overlapped with the source electrode, and the secondconnecting part is overlapped with the drain electrode.
 5. The thin filmtransistor of claim 3, wherein a width of the first connecting part andthe second connecting part is greater than a width of the middle part.6. The thin film transistor of claim 3, wherein a ratio of a length ofthe middle part and a width of the middle part is greater than 1, thelength of the middle part is greater than5 micrometers, and the width ofthe middle part is greater than or equal to 5 micrometers.
 7. The thinfilm transistor of claim 6, wherein the semiconductor layer extends ontothe source electrode and the drain electrode.
 8. The thin filmtransistor of claim 1, wherein the first carbon nanotube layer comprisesa plurality of first carbon nanotubes joined by van der Waals attractiveforce, and the plurality of first carbon nanotubes is disorderlyarranged.
 9. The thin film transistor of claim 8, wherein number of theplurality of first carbon nanotubes in 1 square micrometers is equal toor greater than
 20. 10. The thin film transistor of claim 1, wherein thesecond carbon nanotube layer comprises a plurality of second carbonnanotubes joined by van der Waals attractive force, and the plurality ofsecond carbon nanotubes is disorderly arranged.
 11. The thin filmtransistor of claim 10, wherein number of the plurality of second carbonnanotubes in 1 square micrometers is smaller than or equal to
 1. 12. Athin film transistor comprising: a source electrode; a drain electrodespaced from the source electrode; a semiconductor layer electricallyconnected with the source electrode and the drain electrode; aninsulating layer; and a gate electrode insulated with the sourceelectrode, the drain electrode and the semiconductor layer by theinsulating layer; wherein the gate electrode, the source electrode, andthe drain electrode comprises a plurality of first carbon nanotubes; thesemiconductor layer comprises a plurality of second carbon nanotubes, adistribution density of the plurality of first carbon nanotubes is about20 times that of the plurality of second carbon nanotubes, and number ofthe plurality of second carbon nanotubes in 1 square micrometers issmaller than or equal to
 1. 13. A method of making a thin filmtransistor, the method comprising: providing an insulating substrate;forming a gate electrode on the insulating substrate, wherein the gateelectrode comprises a first carbon nanotube layer with a first filmresistor smaller than or equal to 10 kΩ per square; forming aninsulating layer on the gate electrode; forming a source electrode and adrain electrode on the insulating layer, wherein the source electrodeand the drain electrode are spaced from each other, each of the sourceelectrode and the drain electrode comprises a third carbon nanotubelayer with a third film resistor smaller than or equal to 10 kΩ persquare; forming a semiconductor layer on the insulating layer, whereinthe semiconductor layer is in contact with the source electrode and thedrain electrode, and the semiconductor layer comprises a second carbonnanotube layer with a second film resistor greater than or equal to 100kΩ per square; etching part of the second carbon nanotube layer betweenthe source electrode and the drain electrode to explode a part of theinsulating layer and form a channel, wherein a ratio of a length and awidth of the channel is greater than
 1. 14. The method of claim 13,wherein the providing the first carbon nanotube layer is same as theproviding the third carbon nanotube layer, and the providing the firstcarbon nanotube layer comprises following steps: providing a firstsubstrate and pre-treating the first substrate; depositing a firstcatalyst layer on the first substrate; locating the first substrate withthe first catalyst layer in a reactor; and inputting a carbon sourcegas, and growing the first carbon nanotube layer in a steppedtemperature manner via chemical vapor deposition, wherein the depositingthe first catalyst layer on the first substrate comprises: mixingferritin and water with a volume ratio to obtain a first ferritinsolvent, wherein the volume ratio of ferritin and water ranges fromabout 1:100 to about 1:20; and immersing the first substrate into thefirst ferritin solvent.
 15. The method of claim 14, wherein the growingthe first carbon nanotube layer in the stepped temperature mannercomprises: oxidizing a plurality of catalyst particles of the firstcatalyst layer in air atmosphere; reducing the plurality of catalystparticles in hydrogen or ammonia atmosphere; and inputting the carbonsource gas under about 850 degrees centigrade to about 950 degrees for agrowing time and growing single-walled carbon nanotubes, wherein thegrowing time of the first carbon nanotube layer ranges from about 20minutes to about 2 hours.
 16. The method of claim 13, wherein theproviding the second carbon nanotube layer comprises following steps:providing a second substrate and pre-treating the second substrate;depositing a second catalyst layer on the second substrate; locating thesecond substrate with the second catalyst layer in a reactor; andinputting a carbon source gas, and growing the second carbon nanotubelayer in a stepped temperature manner via chemical vapor deposition,wherein the depositing the second catalyst layer on the second substratecomprises: mixing ferritin and water with a volume ratio to obtain asecond ferritin solvent, wherein the volume ratio of ferritin and waterranges from about 1:8000 to about 1:1000; and immersing the secondsubstrate into the second ferritin solvent.
 17. The method of claim 16,wherein the growing the second carbon nanotube layer in the steppedtemperature manner comprises: oxidizing a plurality of catalystparticles of the second catalyst layer in air atmosphere; reducing theplurality of catalyst particles in hydrogen or ammonia atmosphere; andinputting the carbon source gas under about 850 degrees centigrade toabout 950 degrees for a growing time and growing single-walled carbonnanotubes, wherein the growing time ranges from about 30 seconds toabout 5 minutes.